It is possible to cause phase transition of a phase change material including a chalcogenide element from a crystalline state to an amorphous state at relatively low temperatures such as about 600° C., or from an amorphous state to a crystalline state at lower temperatures. Such a phase change material is used for media such as a compact disk (CD) and a digital versatile disk (DVD). The electrical resistance of this material varies greatly between cases where the material is in a crystalline state and where the material is in an amorphous state. Therefore, studies have been in progress for the purpose of using the above characteristic to utilize the material as a storage material for a semiconductor memory, and some studies yield practical applications. Since the memory has a simple structure, the memory is thought to be a potential candidate for an ultra-high integrated non-volatile memory.
On the other hand, phase change memories currently practically used are still inferior to flash memories, which are widely-used semiconductor non-volatile memories, in terms of high integration. For example, in a case where a memory cell array is configured by a conventional phase change memory cell illustrated in FIGS. 3(a) and 3(b), and when the minimum working size is denoted by F, an area per one cell is 8F2 even when ideal design is realized. In comparison thereto, an area per one cell in the most advanced NAND-type flash memories is 4F2, and an occupied area per one bit is reduced to 2F2 by further employing multi-level technology.
In order to realize a large-capacity phase change memory which replaces a flash memory expected to have a limitation in miniaturization thereof for the purpose of reducing bit cost of SSD expected to have a large market in a mass storage field, it is necessary to use not the conventional phase change memory illustrated in a sectional view of an element in FIGS. 3(a) and 3(b), but a structure suitable for miniaturization. Specifically, as illustrated in FIG. 4(b), a chain-cell structure of a NAND-type circuit system, which is the same as that of an ultra-high integrated flash memory, is used to relatively reduce a region occupied by a memory cell selecting transistor, and then the obtained structure is formed into a three-dimensional structure as illustrated in FIG. 4(a). Consequently, it is possible to realize a phase change memory more suitable for high integration than a flash memory. A sectional structure in FIG. 4(a) and an equivalent circuit in FIG. 4(b) are drawn so as to correspond to each other. A portion enclosed by a dotted line in the figure is one memory cell, and it is possible to build up a structure in which the memory cell is repeatedly piled up in a vertical direction as many times as needed. Details of the structure illustrated in FIGS. 4(a) and 4(b) are described in a literature, 2012 Symposium on VLSI Technology Technical Digest of Technical Papers, p. 35. However, this is an application example of the present invention and is different from the essence of the present invention. Therefore, a detailed description thereof will be omitted.
In order to realize the three-dimensional chain-cell structure, it is necessary to uniformly deposit an ultrathin phase change film having a thickness of several nanometers on a side wall of a fine deep hole or a deep groove formed in a vertical direction of a semiconductor substrate. As a method for forming a uniform and flat phase change thin film on a substrate having such a complex stereoscopic structure, a metal organic chemical vapor deposition method (MOCVD method) is proposed.